Ceramic composites reinforced with ceramic fibers have been known for many years for their strength and toughness. The high toughness of these composites has been characteristically associated with limited bonding between the fibers and the matrix in order to give substantial fiber pull-out as a major source of toughening. On the other hand, brittle failure in unsuccessful composites and resultant flat fractures with essentially no fiber pull-out have been attributed to strong bonding between the fibers and the matrix.
Coatings may be applied to the fibers to limit bonding and chemical reactions with the matrix if the coating has suitable coherence to the fibers and does not spall off. Prior art ceramic fiber composites with metal coated fibers are suitable only for low temperature processing and use because of the relatively low melting point of the metal coatings.
U.S. Pat. No. 4,642,271 discloses ceramic composites suitable for high temperature processing and use. This patent discloses ceramic composites containing ceramic fibers coated with boron nitride and disposed in ceramic matrices. The coating on the ceramic fibers limits bonding between the fibers and the matrix, protects fibers from mechanical damage during handling and processing, provides chemical protection from attack by the matrix during processing, and limits oxidation embrittlement of the ceramic fiber composites.
Ceramic fiber composites have been developed to take advantage of the attractive properties of ceramics while minimizing their brittleness. The attractive ceramic properties of ceramic fiber composites include use temperatures of up to about 1000.degree. C., high temperature structural integrity and high temperature corrosion resistance. The need for high operating temperatures in aircraft engines and reduced emission requirements eliminate the majority of other materials for components in military aircraft and combustor components in advanced designs for both commercial and military aircraft engines. Additionally, the approximately 50% higher specific fracture energy of ceramic fiber composites in comparison to steel makes ceramic fiber composites an excellent candidate for armor superstructures of naval vessels, tanks and other fighting vehicles. Nevertheless, because ceramic fiber composites are more expensive, they have not replaced steel in these applications.